The present invention is concerned with a valve arrangement for brake systems having electronic antilocking and traction slip control systems and which are to be used with automotive vehicles. Such brake systems include an auxiliary force-supported two-circuit brake pressure generator to the brake circuits of which are respectively connected one front wheel and one rear wheel, preferably in diagonal brake circuit division, and also include an auxiliary pressure supply system which, in antilocking control, generates a controlled auxiliary pressure proportional to the pedal force and, in traction slip control, generates a predetermined auxiliary pressure. By way of a valve arrangement, in controlled braking operations, the auxiliary pressure supply system in lieu of the brake pressure generator, through check valves, is connected to the wheel brakes. In each brake circuit, in the pressure fluid conduit leading from the brake pressure generator to the wheel brakes and, in the pressure fluid conduit leading from the auxiliary pressure supply system to the wheel brakes, respectively, one multi-way valve, open for fluid passage in the resting position, is provided. By way of the valve arrangement, in the control phases, the brake pressure is controllable in response to the rotational pattern of the vehicle wheels and/or to the speed of the motor vehicle.
A valve arrangement of this type has been suggested wherein respectively one front wheel and one rear wheel, through multi-way valves open in the resting position, are connected to the two-circuit brake pressure generator, e.g. to a tandem master cylinder coupled ahead of which is a vacuum brake force booster (patent application No. P 36 23 149.5). Connected behind the said valve, i.e. between the outlet of the said valve and the wheel brakes, through a multi-way valve and a series-connected check valve, is the auxiliary pressure supply system such that, after blocking the fluid pressure conduit from the brake pressure generator and after reswitch of the valves, by way of which the wheel brakes are connected to the brake pressure generator, only auxiliary pressure can be fed to the wheel brakes. Moreover, in the prior known valve arrangement, in the pressure fluid conduit leading to the non-driven wheel, another 2-way/2-position valve open in the resting position, is provided which, in the traction slip control phase, is switched to the blocking position so that, in the traction slip control phase, auxiliary pressure is fed only to the driven wheel. Moreover, during antilocking control, the brake pressure, in the wheel brake of the non-driven wheel, can be kept constant, with the pressure in the wheel brake of the second wheel in communication with the same hydraulic circuit being variable.
A "separating valve" provided in the pressure fluid conduit leading to the non-driven wheel, also is already known from W. German published application DE-OS 34 07 538 describing a brake system comprising a master cylinder and a hydraulic brake force booster. In the antilocking control phase, pressure fluid at controlled pressure is discharged from the brake force booster and dynamically fed to the master cylinder. For traction slip control, the pressure fluid conduit leading to the driven wheel is reswitched for direct connection to the hydraulic accumulator of the auxiliary pressure source.
A disadvantage with these state-of-the-art systems and valve arrangements, respectively, resides in that, owing to the restriction to two hydraulically separated circuits and the provision of the valve required for traction slip control in the pressure fluid connecting conduit leading to the non-driven wheel in the pressure build-up phase, it is only possible to feed an elevated brake pressure into the wheel brake of the driven wheel. However, during deceleration of an automotive vehicle, after releasing the gas pedal, the driven wheels are already decelerated by the motor drag moments. It would be advantageous for the brake slip control if in that brake phase, a higher brake pressure were applied to the non-driven wheels than to the driven wheels.